This invention relates to a trellis encoded, narrowband FM system (NBFM) with spectral efficiency and low bit error rate.
1. Description of the Relevant Art
Trellis encoding has evolved primarily into phase and amplitude modulation signalling schemes but not into frequency modulation signalling schemes. A trellis code is a technique for encoding a data stream into a sequence of real vectors that are transmitted over a noisy communications channel. Using phase modulation, two dimensional signal spaces are easily obtainable for generating vectors since in-phase and quadrature-phase signals can be generated with sine and cosine signals. Similarly, quadrature amplitude modulation (QAM) takes advantage of in-phase and quadrature-phase signals, and varying amplitudes of a signal.
The mapping of two dimensional signal space is well developed in the prior art for PSK and QAM signalling schemes. A trellis code using PSK and QAM schemes increases power efficiency and reduces errors. The mapping of two dimensional signal spaces is not well developed, however, for FM systems.
2. Objects of the Invention
A general object of the invention is to increase power efficiency of M-ary digital FM communications using coding.
Another object of the invention is to relax the requirements on hardware capable of modulating and demodulating M-ary channel symbols while still permitting bandwidth efficiency, i.e., a high number of bits per symbols, communications. This objective is achieved by appealing to the fact that embedding messages in high dimensional signal spaces offers greater immunity to noise and other system errors.